PEIRCB. — MANNER OF GROWTH OF A CURRENT. 



529 



up on paper tacked to a screen ; it was generally possible to see the 

 whole of the diagram on the screen besides the bright images of the 

 needle holes, and to reproduce this diagram much enlarged upon 

 the paper. With a given battery, and a given current with a given 

 condition of the iron, it was always easy to get any desired number of 

 records which, when superposed upon the screen, were practically 

 indistinguishable. 



Figure 11 shows a series 17 of building-up curves from a 15-kilowatt 

 transformer very kindly placed at my disposal by Mr. S. E. Whiting. The 

 finely laminated core of this transformer has a cross-section area of 108 

 square centimeters. The same magnetizing coil of about 340 turns was 

 used throughout, but the electromotive force of the storage battery in the 



Figure 12. 



coil circuit could be changed so as to give the current the desired strength. 

 In curve (1) the final intensity of the current was about 3 amperes, in 

 curves (2) and (3) it was 1.5 amperes, and in curves (4) and (5), 0.75 

 amperes. With each value of the electromotive force the steady current 

 was sent through the coil, first in one direction and then in the other, 

 for a number of times, and was then interrupted for a few moments pre- 

 vious to taking the photographic record from the oscillograph. Curves 

 (1), (2), and (4) were obtained when the current in the coil had a direc- 

 tion opposite to that of the next preceding current; curves (3) and (5), 

 when the current had the same direction as the preceding one. For a 

 current of 0.37 amperes the reversed-current-curve had lost its points of 



17 For a large number of similar curves, see Professor T. Gray's paper in the 

 Philosophical Transactions, Vol. 184. 

 vol xli. — 34 



